This invention relates to a spark plug for an internal combustion engine, and particularly to such a plug having a grounded side electrode.
A conventional spark plug consists of a shell threaded to fit a hole in a cylinder head, a center and a side electrode which define a spark gap, and an insulator which serves as a bushing, for efficiently conducting high voltage from an outer terminal to the spark gap exposed in the combustion chamber. Usually, the shaped side electrode is welded to the shell, and then bent to form a spark gap facing the tip of the center electrode. To deal with electrode wear, a multi-pole spark plug fitted with a plurality of L-shaped side electrodes is available. Also available is a spark plug fitted with a flat plate that has a hole in the area facing the tip of the center electrode, and which is connected to the shell by a plurality of legs. An example of such a plug is shown in FIGS. 3(A) and 3(B), wherein the side electrode is generally indicated at 1. It is made of a conductive metal such as a Ni base alloy, and consists of a flat plate portion 3 with a center hole 2 facing the center electrode 5, and a plurality of legs 4 which are connected to the shell 6 to form a unitary assembly.
The spark plug shown in FIG. 3 has been developed in an attempt to improve the durability and anti-vibrational properties of the side electrode, and to attain good firing conditions. Good firing conditions are attained if the difference between the diameter of the center electrode and that of the hole in the side electrode is no smaller than 0.7 mm. However, durability tests conducted on this spark plug have revealed that a marked increase in discharge voltage occurs as a result of cyclic use. The durability tests have also indicated the possibility of cracking of the area between the periphery of the flat plate and the center hole in the side electrode.